lecture notes chem2002 lecture 4 2013 2014

8/11/2019 Lecture Notes CHEM2002 Lecture 4 2013 2014

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CHAPTER FOURProcess Creation

(Input/Output Structure &Economic Potential) 1



Input Information

Reactions and reaction conditions

Desired production rate

Desired product purity or data on $ vs purity

Raw materials and cost

Reaction and catalyst deactivation rates

Processing constraints

Plant/site data

Physical properties of all components Safety, toxicity, & environmental impact of materials

Cost data for by-products, equipment, utilities2



Reaction

Stoichiometry of all reactions occurringTrace by-products can build up in recycle loops, hence all must be

known to synthesize a separation train. Overlooking side reactionsalmost always leads to large economic penalties.

Temperature and pressure ranges

Phases of the reaction system(s)

Product distribution vs. conversion

Conversion vs. space velocity

Catalyst state (homogeneous, slurry, packed bed, powder,etc.), deactivation rate, regenerability and method

3



Selection of Reaction Path by Economic

Potential 1/7

There are typically a variety of reaction paths available toa given product. Paths that use the cheapest raw materials

(commodity chemicals) and produce fewest by-products

are preferred.

Early in the design process, decisions can be made based

on the economic potential (EP) of the process, where the

EP is the difference in value between the products and the

reactants.

4




Potential 2/7

Path 1

C2H2 + HCl C2H3Cl

Path 2

C2H4 + Cl2 C2H4Cl2

C2H4Cl2 + C2H3Cl + HCl

Path 3C2H4 + ½O2 + 2HCl C2H4Cl2 + H2O

C2H4Cl2 + C2H3Cl + HCl5




Potential 3/7

Cost & MW data for materials in Paths 1-3:

Material MW value

Acetylene 26 $0.94/kgChlorine 71 $0.21/kg

Ethylene 28 $0.53/kg

HCl 36 $0.35/kg

Vinyl chloride 62 $0.42/kg

6




Potential 4/7

Path 1

C2H2 + HCl C2H3Cl

Economic Potential for Path 1 =

(62 x 0.42) – (26 x 0.94 + 36 x 0.35) = -$11.0/kmol VCM

7




Potential 5/7

Path 2

C2H4 + Cl2 C2H4Cl2



(62 x 0.42 + 36 x 0.35) – (28 x 0.53 + 71 x 0.21) = $8.89/kmol VCM

Assuming HCl by product cannot be sold,

(62 x 0.42) – (28 x 0.53 + 71 x 0.21) = -$3.71/kmol VCM

8




Potential 6/7

Path 3

C2H4 + ½O2 + 2HCl C2H4Cl2 + H2O



(62 x 0.42) – (28 x 0.53 + 36 x 0.35) = -$1.40/kmol VCM

9




Potential 7/7

Path 1 = -$11.0/kmol VCM

Path 2 = $8.89/kmol VCM

HCl unsaleable = -$3.71/kmol VCM

Path 3 = -$1.40/kmol VCM

10



Reaction

Reaction data is critical to the input/outputunderstanding of the flowsheet

Using isobutylbenzene manufacture as an example,

build the input/output structure

Toluene (T) Propylene (P)isobutylbenzene (IBB)

+

Na/K

HighFlo

11



I/O Use of Reaction Information

Level I

I/O structure

Toluene (T) Propylene (P)isobutylbenzene (IBB)

P

T

IBB

P, T

NBB

Na/K/Hi-Flo

deactivated Na/K/Hi-Flo

+Na/K

Hi-Flo

N-butylbenzene (NBB) 12



Level I Decision:

Production Rate

Economies-of-scale are most favorable for large plants

Maximum size may be limited by the maximum

size of a piece of equipment (rail shipping doeshave size limits)

Development of new technologies necessary

Changing marketCurrent market share vs. plant size of

competition13



Level I Decision:

Batch vs. Continuous

Production rate (>106 continuous)

Market forces

Seasonal products, allowing equipment to be used for

other purposes during off-seasonOperational problems

Reaction kinetics

Handling low capacity slurries

Fouling vs. clean-outs

Multiple operations in a single vessel14



Level I Decision:

Batch vs. Continuous

Select process units needed Choose interconnections among units

Identify alternatives to be considered

List dominant design variables

Estimate optimum processing conditions Determine best alternative

Determine which units should be batch

Determine which processing steps should be carried out in asingle vessel

Determine when it is advantageous to operate parallel batch unitsto improve scheduling

Determine the amount of intermediate storage and surge capacitythat is needed

15



Level I Critical Information:

Plant and site data

Plant must be compatible if built on an existingsite.

Battery-limits and costs to consider

UtilitiesFuel supply

Steam pressure levels

Cooling water inlet/outlet temperatures

Refrigeration levelsElectric power

Waste disposal facilities16




Plant and Site Considerations

Plant LocationRaw Material Availability

purchase price, distance from supply, transport expenses, reliability of

supply, purity

Product & by-product Markets or Intermediate DistributorsEnergy Availability

power (hydroelectric), fuel (coal, oil)

Climate

cold requires shelters for equipment, line tracing; heat may require

cooling towers or refrigeration systems; humidity levels must be

considered

Transportation Facilities

channels, railroads, highways (2 are desirable)17





Plant LocationWater Supply

cooling, washing, steam generation, raw material; properties:

temperature, mineral/silt content, bacteria content, supply/treatment

cost

Waste Disposal

landfills, hazardous treatment

Labor Supply

pay scale, hour restrictions, competing industry, skill level

Taxation & Legal restrictions

state & local tax rates on income, unemployment insurance, property;

local zoning regulations, codes, transportation facilities18





Plant LocationSite characteristics

tract topography, soil structure, acreage cost, building costs, cost

of living, future expansion

Flood & Fire Protection

regional natural events (flood, hurricane, earthquake), assistance

from local fire departments

Community

cultural facilities, churches, libraries, schools, civic theatres,

concert associations, recreation...

19





Plant Layout new site development vs. site addition

type/amount of products being produced

type of process and control technique

economic distribution of utilities and services

building types and code requirements

health and safety consideration

waste-disposal requirements

space available and required

road and railroad location possible future expansion

storage facilities

materials being handled20



Build Input/Output Flowsheet from

Reaction/Plant Data

outin

21



Input/Output Synthesis

IBB: SX

NBB: (1-S)XP: Q-x

T: 1-X

X = conversionS = selectivity

I/OFlowsheet

T (1 mole basis)

P (Q)

build lowest level I/O Flowsheet using reaction data

build overall material balance

22




reactor

T (1 mole basis)

P ( ) separationtrain

P: hP(Q-x)

T: hT(1-X)

IBB: hISX

NBB: hN(1-S)X

h = separation efficiency

• Include most basic separation scheme

23



Level II Decisions

Should we purify the feed streams prior to entering the process?

Should we remove or recycle a reversible by-product?

Should we use a gas recycle and purge stream?

Should we not bother to recover and recycle somereactants?

How many product streams will there be?

What are the design variables for the input/output

structure, and what economic trade-offs are associatedwith these variables?

24

L l II D i i



Level II Decision:

Purification of Feeds

If a feed impurity… is not inert and is present in significant quantities, remove it (it may

lead to raw-material losses and a more complicated separationsystem to remove the additional by-products).

is present in a gas feed, as a first guess, process the impurity.

in a liquid feed stream is also a by-product or a product component,usually it is better to feed the process through the separation system.

is present in large amounts, remove it.

is present as an azeotrope with a reactant, process the impurity.

is inert, but easier to separate from the product than the feed, process the impurity.

is a catalyst poison, remove it.

25




IBB: SX

NBB: (1-S)X P: (QfP-fTx)

T: (1-X) fT

I: I(1-fI)

= separation

factor

impurityseparation

T (1 mole basis)

P (Q)impurity (I)

reactor

P: Q(1- fP)

T: 1- fT

I: IfI

I: I(1-fI)

P: QfP

T: fT

• Consider need for impurity separation from feed

26

L l II D i i



Level II Decision:

Purification of Feeds

If the method for dealing with a feed impurity isunclear, list the opposite decision as a process

alternative.

Feed purification has economic trade-offs that preclude any simple design criterion that indicate

the correct decision.

Capital cost of a preprocess purification system

Raw material yield gains/losses

27

L l II D i i R & R l



Level II Decision: Recover & Recycle

Reversible By-Products

A + B

C + D2D E + B

Since the second reaction is reversible, we couldrecycle E back to the reactor and let it build up inthe loop until it eventually reached an equilibriumlevel.

If we recycle E, we must oversize all of the processequipment in the loop.

If we remove it from the process, we pay the economic penalty of increased raw-material cost. 28




reactorT, P separationtrain

P: hP(Q-x)

T: hT(1-X)

IBB: hISX

NBB: hN(1-S)X

recycle structure $

Consider economics of reactant recovery/recycle

29

Le el II Decision



Level II Decision:

Gas Recycle & Purge

If a Level 1 I/O flowsheet containsa light reactant, AND

a light feed impurity or reaction by-product

It is common practice to use a recycle/purge scheme to

recover the value of the reactant while removing theimpurity or by-product.

A component is “light” if it has a lower boiling point than

propylene (-48°C)

Propylene is the selected cut-off because lower-boiling componentsnormally cannot be condensed at high pressure with cooling water,

refrigeration would be needed.30




reactorT, P separationtrain

P: f hP(Q-x)

T: hT(1-X)

IBB: hISX

NBB: hN(1-S)X

f = split fraction

Consider economics of gas recycle/purge stream

P: (1-f)hP(Q-x)

31



Number of Product Streams

To specify the number of effluent streams,list all components that will leave the process

Classify each component and assign a destination code

Order the components by their destinations

Number of groups of all but recycle streams is then considered to bethe number of product streams

Initally, follow the guideline “It is never advantageous to separate

two streams, then recombine them later”

Azeotropes will affect this process

Solids require modification to the rules

32



Number of Product Streams

Component Destination Component DestinationA Waste F Primary product

B Waste G Recycle

C Recycle H Recycle

D Fuel I Valuable by-product

E Fuel J Fuel

effluents

1) A + B: to waste (do not separate, mix to sewer)

2) D + E: to fuel (do not separate, mix to burn)3) F: primary product (to storage for sale)

4) I: valuable by-product (to storage for sale/use)

5) J: to fuel (J must be separated from other fuels to recover products)33



Example Synthesis1/12

Benzene production from Toluene

Desired C6H6 production rate

PB = 265 mol/hr

OH + H2 + CH4

+ + H2

CH3

34



Example (Synthesis)2/12

Number of Process Streams

component boiling pt. destination

H2 -253°C recycle/purge

CH4 -161°C recycle/purge

benzene 80°C primary product

toluene 111°C recycle

diphenyl 253°C fuel35




Level I I/O Flowsheet

process

H2, CH4

H2, CH4

toluene

benzene

diphenyl

36




Economic PotentialEP = product value

+ by-product value

– raw materials cost, $/yr

EP = benzene value

+ fuel value of diphenyl

+ fuel value of purge

- toluene cost- makeup gas cost

42




Using the following values:Benzene, $0.85/gal $9.04/mol

Toluene, $0.50/gal $6.40/mol

Hydrogen, $3/ 1000 ft3 $1.14/mol

Fuel = $4/106 Btu

Hydrogen, 0.123 x 106 Btu/mol

Methane, 0. 383 x 106 Btu/mol

Benzene, 1.41 x 106 Btu/mol

Toluene, 1.68 x 106 Btu/mol

Diphenyl, 2.688 x 106 Btu/mol43




0 0.2 0.4 0.6 0.8

conversion

$ / h r

At high conversions,

profitability losses occur

due to selectivity losses

to diphenyl production.

A similar analysis would

show profitability loss occurs

also at high purge H2

concentrations.

Hence, expect X and yH2 to be optimization variables

44

12/12




Process alternatives to be consideredPurify hydrogen feed stream

Recycle diphenyl to extinction

Purify the H2

recycle stream

45

lecture notes chem2002 lecture 4 2013 2014

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